Motion simulators are well known in the art. The Stewart platform (or hexapod) is a well known form of simulator which moves a platform relative to a base. Hexapods have six linear actuators arranged to move the platform in six degrees of freedom (three linear, three rotational) relative to the base depending on which actuators are used in combination. The translational degrees of freedom are commonly known as surge (horizontal movement in the direction of travel), sway (horizontal movement perpendicular to the direction of travel) and heave (vertical motion). The rotational degrees of freedom are known as roll (rotation about an axis parallel to the direction of travel), pitch (rotation about a horizontal axis perpendicular to the direction of travel) and yaw (rotation about a vertical axis).
Hexapods have finite workspaces defined by the maximum and minimum excursion of the platform, which in turn is defined by the limit of travel of the actuators. For larger workspaces requiring further platform movement in any given degree of freedom, it is known to provide longer hexapod actuators. Although this may achieve the desired result, it substantially increases the cost of the simulator (longer linear actuators are significantly more expensive than short ones), and can sometimes decrease its inherent stiffness. In some cases, hexapods are simply unsuitable for the required degree of excursion.
Stiffness is an important property of the simulator, because it minimises undesirable vibration and oscillation of the platform, which would otherwise provide false accelerations, and forces on the subject. In known Stewart platforms there is therefore a trade off between maximum platform excursion and stiffness.
There are various simulations which require a high excursion, or degree of travel, in a specific rotational degree or degrees of freedom. This can be used to simulate gravity or radial accelerations. For example, fuel tank testing, battery testing, fuel metering system testing, inertia measurements of equipment, testing instruments, fixation methods testing, equipment containing/depending on liquids or magnets and any equipment that requires an artificial horizon all require potentially large platform movements in the global roll and pitch degrees of freedom. Providing a hexapod with long stroke actuators would provide the required functionality to a certain extent, but not in all cases. Large hexapods would also provide functionality which is not required—namely additional travel in the remaining four degrees of freedom.
As such, there is competing requirement to provide a stiff, compact and inexpensive simulator on the one hand, and to provide additional movement in the roll and pitch degrees of freedom on the other hand.